Electrostatic coating system and methods



April 24, 1962 TAMOTSU WATANABE ELECTROSTATIC COATING SYSTEM AND METHODS Filed Jan. 50, 1969 2 Sheets-Sheet 1 INVENTOR.

TR MOTS U WATANABE RT TOR MESS April 1962 TAMOTSU WATANABE 3,031,337

ELECTROSTATIC COATING SYSTEM AND METHODS Filed Jan. 30, 1959 2 Sheets-Sheet 2 INVENTOR.

TAMOTSU WHTHNHBE BY W kI IKW HTTORNEjS surface of the article or work pieces to be coated.

United States 3,031,337 ELECTROSTATIC OATIN G SYSTEM AND METHODS Tamotsu Watanabe, 28 Yasukata-cho, Ohm-Ward, Tokyo, Japan Filed Jan. 30, 1959, Ser. No. 790,138 8 Claims. (Cl. 117-493) This invention relates to a method of and apparatus for applying atomized liquid coating materials onto work pieces. More particularly, the invention relates to a novel method of and apparatus for atomizing liquid coating materials by means of compressed air and then moving the atomized particles onto work pieces by means other than the velocity of the compressed air discharged from the atomizing nozzles, such as by means of elec- 'coating'material particles on a work-piece surface if the atomized particles were given neither any direction nor any velocity, particularly if the particles merely floated in the air or remained in a quiescent state. However, if it is desired to move such quiescent atomized particles in a desired direction by means of some secondary force, it is self-evident that the secondary force can be applied most easily and effectively if the atomized particles are substantially completely quiescent.

One of the features of the present invention is the use of suitable atomizing nozzles that spray and atomize any desired coating or painting material through extremely small diameter tips, using compressed air at as low a pressure as possible.

Another feature of the invention is that the nozzles are movable relative to the work piece surface to be coated, as the atomized particles, if directed out of fixedly positioned nozzles, would have a certain velocity and a fixed direction relative to the nozzles due to the use of compressed air to atomize the'coating material. In accordance with the present invention, the atomized particles, as discharged from the nozzles, are caused to have substantially zero velocity in the air, so that the cloud of discharged atomized particles will be substantially completely quiescent and remain standing in the air when the nozzles are moved in a direction and at a velocity to off-set the discharge velocity. Specifically, if the nozzles are moved rearwardly with respect to their direction of discharge at a velocity equal to the discharge velocity, the projected particles will float without any specific velocity or fixed direction of movement in a plane parallel to the direction of nozzle movement and are in a state to be readily affected by other forces. Accordingly, when an electric charge or an electrostatic potential gradient is applied to the particles to direct the same toward the work piece surface to be coated, it is possible to deposit nozzles may be of known construction, designed to finely atomize liquid coating material by means of compressed air. The circular head is constructed and arranged for connection to a source of compressed air and to a source of liquid coating material, and so as to deliver the com- 3,931,337 Patented Apr. 24, 1962 pressed air and liquid coating material uniformly to the nozzles. Means, such as an electric motor, is provided to rotate the head about its axis, and the nozzles are so positioned on the periphery of the head that, as they are revolved, their discharge axes remain in a plane substantially perpendicular to the axis of rotation of the head.

In further accordance with the invention, the head is rotated to move the nozzles in a direction opposite to the direction of discharge of the atomized material from the nozzles, and at a peripheral velocity substantially equal to the discharge velocity of the compressed air containing the finely atomized material. Thus, when the atomized liquid coating material is discharged from the nozzles, which latter have 'a rearward motion at a velocity substantially equal to the rate of discharge of the atomized particles therefrom, the discharged particles remain in a quiescent state, seemingly being suspended in air in an area close to the nozzles, and having substantially no velocity or positive direction parallel to the plane of movement of the nozzles. Thus, the quiescent state of the atomized particles is achieved by the reduction of the absolute velocity of the compressed air discharged from the nozzles to substantially zero by the imparting to such dischar ed compressed air of a counterforce or velocity having a direction opposed to the direction of discharge, this being effected byrevolution of the nozzles by the circular head in a direction counter or opposite to the discharged direction of the nozzles.

The atomized particles, now in a quiescent state, are given positive direction toward the surfaces to be coated in any desired manner, and preferably by placing an electrostatic charge upon the particles to create an electrostatic field or potential gradient between the nozzles and the work piece surfaces to be coated. Thereby, the quiescent particles are caused to migrate toward the work, the work preferably being grounded.

For an understanding of the principles of the invention, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawings. In the drawings:

FIG. 1 is a front elevational view of a head, having suitable atomizin nozzles mounted thereon, and embodying the invention;

FIG. 2 is a side elevational view of the head and nozzles shown in FIG. '1, as arranged in relation to a surface to be coated;

FIG. 3 is a plan view showing the coating pattern provided by the method of the present invention; and

FIGS. 4 and 5 are somewhat schematic plan views illustrating the operation of the invention.

Referring to FIG. 1, an atomizing head 2 is secured to the forward end of a tubular shaft 1, which is rotatably mounted and has an axial passage 4. A plurality of nozzles 3 is mounted on the periphery of head 1, the inlet axes of these nozzles being located in a common diametric plane through head 2 and shaft 1. The discharge ends of nozzles 3 are adjustable about the inlet axes of these nozzles so that the material discharged therefrom may have any direction of discharge desired, and may be at any angle to such diametric plane. The nozzles 3 are designed to discharge compressed air carrying any selected liquid painting or coating material.

The compressed air is fed to the axial aperture 4 of tubular shaft 1, and the painting material is fed through its supply line 8 to a chamber 5 on the back end of head 2. A suitable difference in electric potential is provided in the zone between the head 2, carrying the nozzles 3, and the articles 6 which are to be coated. While it is preferable that the head Z be charged with a high voltage directly from the high voltage source 7, alternatively an electrode can be positioned in the region at the back of the head 2.

It is also possible to create special conditions of ionization in the compressed air system or in the inner walls of the nozzle tips. In any case, if a liquid coating or painting material fed to the head 2 from a suitable source, such as the supply system 8, is projected from the nozzles with a velocity of (V) by compressed air, and if a velocity of (Vo) equal to the discharge velocity (V0) be imparted to the nozzles in a direction substantially opposed to the discharge direction, the spray at the tips of the atomizing nozzles will therotically have zero velocity in air. In brief, while the atomizing nozzles spray the material at a velocity of (V0), the retrograde movement of the nozzles results in the atomized material particles 9 being projected at an effective velocity of (V0- V0), or zero, so that the particles are substantially quiescent in the air. Because the particles are sprayed into the electric field in front of the surface 6 to be coated, they will immediately be affected by theelectric charge and be deposited on the surface to be coated.

In practice, various arrangements of apparatus may be used to carry out the invention, but the embodiment shown in FIG. 1 by way of example, and including a group of nozzles on the rotating head 2, has been found to be very satisfactory in operation both by practical tests and by experimentation. In this embodiment, the nozzles are rotated, as shown by the arrow A, together with the head 2, around the axis of the shaft 1, while the atomized particles are projected in the direction B from the tips of nozzles 3. Therefore, it will be apparent that, in order that the atomized particles, as discharged, have substantially zero velocity, there must be a substantial balance between the discharge velocity and the peripheral velocity of the nozzles. From this it may be seen that, when the head 2 is rotated at a velocity such that the peripheral velocity of the nozzles is substantially equal and opposite to the discharge velocity of the atomized material, foggy particles, in a state of zero velocity, will remain in a cloud around the head. Thus, it is very easy to alter the direction of the particles to any desired direction by applying a potential gradient, with eflicient results insofar as the coating is concerned.

All prior art methods generally known as electrostatic coating or painting have heretofore been based on the fundamental principle of electrostatic atomization of the material plus electrostatic deposition of the material on the surfaces to be coated, and arrangements involving the rotation of the atomizing head as an electrode have been suggested and used in order to utilize centrifugal force.

'In such known coating methods however, inasmuch as they rely on accomplishing both atomization and deposition by electrostatic force, extremely high electrostatic voltages are necessarily required, and these high voltages naturally cause the deposition of the particles to be concentrated on corners or edge portions, or adjacent corners or edge portions, of the surfaces to be coated. This is a result of acute corona phenomenon, and it is difficult, if not impossible, to obtain effective uniform coating on reentrant or dented portions of the surfaces of flat sheets. However, if the voltage is reduced, the atomized particles will become much more coarse, with resultant inferiority of the applied coating. These drawbacks have been discovered through practical use of known apparatus.

Referring to FIG. 5, in the painting of articles 61, 62 and 63, rotating as shown by the arrows D and suspended from a conveyor 10, the articles move in the direction indicated by the arrow C. In systems of coating heretofore used, the plane surface of an article 62 passing directly in front of a coating or spray head 21 is not effectively coated, as many of the atomized coating material particles discharged from the head 21 are deposited on corner edge portions P and Q of the articles 61 and 63 positioned on either side of the article 62. To prevent this condition, it has hitherto been necessary to considerably increase the spacing of the articles along the conveyor '10, or to increase the hanger pitch. This results in substantial lengthening of the conveyor and collateral or associated drying equipment.

Furthermore, the spray pattern provided by conventional electrostatic coating methods is generally in the form of a doughnut-shaped circle, which results in nonuniformity of the deposited coating. If a combination apparatus is used wherein the electrode is rotated along with the atomizing head, the doughnut-shaped pattern will be even more pronounced. On the other hand, atomized coating material particles subjected to centrifugal forces are influenced by forces of different values, depending upon their sizes and weights, so that the coarser particles are thrown a greater distance and the finely divided particles a lesser distance from the axis of discharge. This results in a lack of uniformity detracting from the finished appearance of the coated particle.

By contrast, with the invention method and apparatus, the coating material spray, comprising very finely and uniformly subdivided particles due to the compressed air atomization, upon becoming electrically charged While the particles are substantially in a stand-still or quiescent state, will effect a deposition of the particles on the work surfaces relatively slowly and gradually and without requiring the application of a high potential gradient. As there is no acute corona effect at the corners or edges of the articles to be coated, there will be a uniform coating applied over the entire surface. Furthermore, in the case of articles passing directly in front of the head of FIGS. 1 and 2, these can be coated effectively with substantial shortening of the hanger pitch, resulting in economy of equipment and apparatus, as well as in increased efficiency of the coating.

The coating pattern 11 provided by the method and apparatus of the invention is in a generally circular shape with a slightly thinner coat at the center of the pattern, as shown in FIG. 3. However, if the articles to be coated pass through the coating zone as indicated by the arrow E, the invention method is effective in providing a uniform and high quality coating. While patterns with a so-called concave coat or with a blank space at the center of the surface to be coated are provided by known methods, coatings provided by the present invention are only slightly thin at the center, so that it is easy to achieve uniform coating with the invention method.

While it is a principle of the present invention that the discharge nozzles have a velocity equal and opposite to the discharge velocity, in the event uniformity of coating is not particularly important, due to the nature of the article or surface to be coated, or when applying a final coating, the nozzles may be adjusted to extend slightly toward the surface to be coated so that the atomized particles have a small velocity component (Vd) toward the surface 6 to be coated. In such case, the velocity of the nozzles is equal and opposite to that component of the discharge velocity parallel to the plane of movement of the nozzles.

It is essential, in known methods of both atomizing and depositing coating materials solely by means of an electrostatic potential, that good coating materials, suitable for electrostatic atomization and deposition, be provided in order to assure proper atomization. If the materials used fail to meet the proper standards for effective electrostatic atomization, the atomization and deposition are adversely affected. Consequently, in electrostatic atomization and deposition methods heretofore used, it has been difficult, if not impossible, to attain satisfactory results using low quality coating material such as commonly used in spray painting wherein only compressed air is used for both atomization and deposition. However, with the present invention using compressed air for the atomization and an electrostatic potential gradient for deposition, finely divided particles are produced by the compressed air atomization and these finely divided particles, having an effective velocity of substantially zero after discharge from the-nozzles, are effectively deposited on the works by means of the electrostatic potential gradient. Thus, with the present invention, low priced and low quality painting materials can be used, which latter cannot be used with processess wherein only electrostatic forces are used for both atomization and for deposition. Consequently, the present invention method is uniformly and universally applicable irrespective of the quality of the coating materials.

In comparison with systems now in use, the efliciency of the present invention has been to be of the order of 104 to 110 considering an efiiciency of 100 as that of known prior art methods. This has been established through numerous tests and experiments. This high efliciency follows as a matter of course from the nature of the present invention wherein the atomization is effected solely by compressed air, to produce uniformly fine particles, and wherein the velocity of the atomized particles, after discharge from the nozzles, is substantially zero so that they may be readily affected by the electrostatic potential gradient for movement toward the surfaces to be coated, the particles not having substantially any velocity in any other direction which must be overcome by the electrostatic force.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

I claim:

1. Apparatus for electrostatically coating work comprising, in combination, discharge nozzles; means for supplying coating material to said discharge nozzles; means for supplying compressed air to said nozzles to atomize the coating material and to discharge streams of coating material particles from the nozzles; means moving the nozzles in a direction opposite to the direction of discharge of the coating material particles and at a velocity substantially equal to the discharge velocity of the coating material particles to provide a substantially quiescent spray of the coating material particles, with the coating material particles having substantially no absolute velocity; and means establishing an electrostatic potential gradient between the nozzles and the Work to move the relatively quiescent coating material particles onto the work to coat the latter.

2. Apparatus as claimed in claim 1 in which said nozzles are mounted on the periphery of a mounting head which is rotatable about an axis in a plane substantially parallel to the surface of the work to be coated, and the coating material and the compressed air are supplied to said mounting head and from said mounting head to said nozzles.

3. Apparatus for electrostatically coating work comprising, in combination, plural discharge nozzles; means for supplying coating material to said discharge nozzles; means for supplying compressed air to said nozzles to atomize the coating material and to discharge streams of atomized coating material particles from the nozzles; means moving the nozzles, during such discharge, in a closed path in a common plane with the axes of the nozzles positioned at a common angle to the plane and directed partially toward the work, and in a direction opposite to the direction of that component of the velocity of the discharged coating material which is substantially parallel to such plane and at a velocity substantially equal to such component; the discharge velocity of the coating material particles having a second and minor component which is perpendicular to the first mentioned component and directed toward the work; whereby to provide a substantially quiescent spray of the coating material particles, with the coating material particles having only such minor component of the discharge velocity in a direction toward the work; and means establishing an electrostatic potential gradient between the nozzles and the work to move the relatively quiescent coating material particles onto the work to coat the latter.

4. Apparatus for electrostatically coating. work comprising in combination, plural discharge nozzles; means for supplyingcoating material to said discharge nozzles; means for suppling compressed air to said nozzles to atomize the coating material and to discharge streams of atomized coating material particles from the nozzles; means moving the nozzles, during such discharge, in a closed path in a common plane, with their discharge orifices trailing, at a velocity substantially equal to the component of the discharge velocity parallel to such plane; whereby to provide a substantially quiescent spray of the coating material particles, with the discharged coating material particles having substantially zero velocity relative to the work in any direction parallel to such plane; and means establishing an electrostatic potential gradient between the nozzles and the work to move the relatively quiescent atomized particles onto the work to coat the latter.

5. In the method of electrostatically coating workpieces by atomized liquid coating material, the improvement comprising atomizing the coating material by discharging it from nozzles by means of compressed air; and simultaneously revolving the nozzles, with their discharge axes in a plane substantially perpendicular to the axis of rotation of the nozzles and spaced from the work-piece surface to be coated, in a direction opposite to the direction of discharge of the atomized coating material and at a velocity substantially equal to the discharge velocity of the atomized coating material; whereby the discharged atomized coating material particles are in a substantially quiescent state immediately around the nozzles and have substantially zero absolute velocity; and establishing an electrostatic potential gradient between the atomized coating material particles and the work-piece surface to move the relatively quiescent coating material particles onto the work-piece surface to coat the latter.

6. In the method of electrostatically coating work-pieces by atomized liquid coating material having a charge of polarity opposite to that on the work-pieces, the improvement comprising atomizing the coating material by discharging it from nozzles by means of compressed air; simultaneously revolving the nozzles, with their discharge axes in a plane substantially parallel perpendicular to the axis of rotation of the nozzles and spaced from the workpiece surface to be coated, in a direction opposite to the direction of discharge of the atomized coating material and at a velocity substantially equal to the discharge velocity of the atomized coating material; whereby the discharged atomized coating material particles are substantially in a quiescent state immediately around the nozzles and have substantially zero absolute velocity; and applying to the quiescent atomized coating material particles a charge of a polarity opposite to that of the charge on the work-piece.

7. In the method of electrostatically coating work pieces by atomized liquid coating material, the improvement comprising atomi-zing the coating material by discharging it from a plurality of nozzles by means of compressed air; and simultaneously moving the nozzles in a closed path in a common plane with the axes of the nozzles positioned at a common angle to the plane and directed partially toward the work pieces; the velocity of the atomized coating material relative to the nozzles having a first component parallel and opposite to the direction of movement of the nozzles in such path, and a second and minor component perpendicular to said first component; the magnitude of the velocity of the nozzles in such path being substantially equal to the magnitude of the first component; whereby the velocity of the atomized coating material relative to the work pieces has substantially no component in any direction parallel to the plane of movement of the nozzles but has a minor component of predetermined magnitude in a direction perpendicular to the plane of movement and toward the work pieces; and establishing an electrostatic potential gradient between the atomized coating material particles and the work pieces to move the relatively quiescent coating material particles onto the work pieces to coat the latter.

8. In the method of electrostatically coating work pieces by atomized liquid coating material, the improvement comprising atomizing the coating material by discharging it from a plurality of nozzles by means of compressed air; simultaneously moving the nozzles in a closed path in a common plane, with their discharge orifices trailing, at a velocity substantially equal to the component of the discharge velocity parallel to such plane; whereby the velocity of the discharged atomized coating material particles relative to the work pieces in any direction parallel to the plane of movement of the nozzles is substantially zero; and establishing an electrostatic potential gradient between the atomized coating material particles and the work pieces to move the relatively quiescent atomized particles onto the work pieces to coat the latter.

References Cited in the file of this patent UNITED STATES PATENTS 1,909,260 Forbes May 16, 1933 2,328,448 Gustafsson et al Aug. 31, 1943 2,334,648 Ransburg et al Nov. 16, 1943 2,466,906 Miller Apr. 12, 1949 2,697,411 Ransburg Dec. 21, 1954 2,773,472 Lamm Dec. 11, 1956 2,780,565 Juvinall Feb. 5, 1957 2,781,280 Miller Feb. 12, 1957 2,860,599 Rice et a1 Nov. 18, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 3,031,337 April 24, 1962 Tamotsu Watanabe It is hereby certified that error appears in the above numbered patant requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 45, for "substantially parallel perpendicular" read substantially perpendicular Signed and sealed this 9th day of April 1963.

SEAL) Attest:

ESTON G. JOHNSON Attesting Officer DAVID L. LADD Commissioner of Patents 

5. IN THE METHOD OF ELECTROSTATICALLY COATING WORKPIECES BY ATOMIZED LIQUID COATING MATERIAL, THE IMPROVEMENT COMPRISING ATOMIZING THE COATING MATERIAL BY DISCHARGING IT FROM NOZZLES, BY MEANS OF COMPRESSED AIR; AND SIMULTANEOUSLY REVOLVING THE NOZZLES, WITH THEIR DISCHARGE AXES IN A PLANE SUBSTANTIALLY PERPENDICULAR TO THE AXIS OF ROTATION OF THE NOZZLES AND SPACED FROM THE WORK-PIECE SURFACE TO BE COATED, IN A DIRECTION OPPOSITE TO THE DIRECTION OF DISCHARGE OF THE ATOMIZED COATING MATERIAL AND AT A VELOCITY SUBSTANTIALLY EQUAL TO THE DISCHARGE VELOCITY OF THE ATOMIZED COATING MATERIAL; WHEREBY THE DISCHARGED ATOMIZED COATING MATERIAL PARTICLES ARE IN A SUBSTANTIALLY QUIESCENT STATE IMMEDIATELY AROUND THE NOZZLES AND HAVE SUBSTANTIALLY ZERO ABSOLUTE VELOCITY; AND ESTABLISHING AN ELECTROSTATIC POTENTIAL GRADIENT BETWEEN THE ATOMIZED COATING MATERIAL PARTICLES AND THE WORK-PIECE SURFACE TO MOVE THE RELATIVELY QUIESCENT COATING MATERIAL PARTICLES ONTO THE WORK-PIECE SURFACE TO COAT THE LATTER. 